Type Traits

TODO: Describe all language-related type traits.

Incomplete list:

• copyable/non-copyable (do we have movable/non-movable?)
• opaque/non-opaque
• known size/unknown size. Note: this might need further classification into implementation-specified sizes known by slangc vs sizes that only the target compiler knows (but sizeof() would still work)
• allowed storage duration: static (globals), block (function locals/params/return)
• serializable/non-serializable (types that can be assigned to interface-typed variables)
• addressable/non-addressable (types that are valid types for pointers)
• etc